1. Field of the Invention
The present invention relates to a selective call system, particularly but not exclusively, to a message transmission system in which long data messages and/or telescript are (or is) transmitted.
2. Description of the Related Art
PCT Patent Specification W094/28685 discloses a paging system which is suitable for sending long data messages at higher rates than is currently normally used and which is an enhancement of, and compatible with, the CCIR Radiopaging Code No 1 alternatively known as the POCSAG paging code. This enhanced paging system, known by the Applicant as the Advanced Paging Operators Code (APOC), has provision for sending address code words and concatenated message code words in cycles having a duration of 6.8 secs. Each cycle comprises a plurality of batches, for example 3 batches of equal duration. Each batch comprises a synchronisation (sync) code word concatenated with n frames, each of which is constituted by m code words.
When transmitting long data messages it is often desired to obtain an indication that a data message has been received successfully and better still to obtain a reply of some sort, however simple. The transmission of acknowledgements to paging messages has been disclosed in the art. In U.S. patent application Ser. No. 08/430518 filed Apr. 27, 1995 it is disclosed to transmit responses, that is acknowledgements/simple replies, as spread spectrum signals. By several pagers having different spreading code sequences, they can all respond simultaneously and each response can be recovered at the base station or paging system controller by multiplying the received signal with each of the respective spreading code sequences. The above-mentioned U.S. patent application Ser. No. 08/430518 also discloses allocating 6 unique spreading code sequences to all the message pagers allocated to the same frame of a batch, with each code sequence having a different meaning, such as:
Code Sequence 1--secondary station in the area for purposes of registration only. PA0 Code Sequence 2--Received last message. PA0 Code Sequence 3--Read message(s). PA0 Code Sequence 4--Answer "Yes". PA0 Code Sequence 5--Answer "No". PA0 Code Sequence 6--Resend last message.
In operation when a system controller has sent a data message to a pager or secondary station, a microcontroller in the addressed secondary station will, if the message has been received and decoded successfully, multiply the reply pulse with, say, Code Sequence 2 and transmit the signal. As the system controller knows to which secondary station a data message has been sent, the protocol will allow a suitable time to elapse before transmitting a further message to that secondary station.
There are other cases where it is desired for a secondary station to send a signal, other than in response to a previous outgoing message, for example to register the presence of a pager in an area covered by a particular base station.
In a selective call system in which data messages are transmitted by a primary station to a secondary station which may roam relative to the primary station and which transmits responses as very low power spread spectrum signals, there is the near/far problem in which strong signals may suppress weak ones. This could mean that a signal which is received relatively weakly at a receiver associated with the system controller may be suppressed by a much stronger signal and in consequence is not detected.